1. Field of the Invention
This invention relates to a scanning optical apparatus and an image forming apparatus using the same, and particularly is suitable for an apparatus such as a laser beam printer or a digital copier having the electrophotographic process which is designed such that at least one beam light-modulated and emitted from light source means is reflected and deflected (deflected and scanned) by deflecting means comprising a rotary polygon mirror or the like, whereafter a surface to be scanned is optically scanned through imaging means having at least one diffracting surface to thereby record image information.
2. Related Background Art
In a scanning optical apparatus in a laser beam printer (LBP) or the like, a beam light-modulated in conformity with an image signal and emitted is periodically deflected by a light deflector comprising, for example, a rotary polygon mirror, and is converged into a spot shape on the surface of a photosensitive recording medium (photosensitive drum) by imaging means having an fθ characteristic and that surface is optically scanned to thereby effect image recording.
Further, various scanning optical apparatuses having a diffracting surface on a portion of imaging means (scanning optical means) are proposed, for example, in Japanese Patent Application Laid-Open No. 10-68903, etc. In Japanese Patent Application Laid-Open No. 10-68903, an optical element having a refracting portion (refracting surface) and a diffracting portion (diffracting surface) is used as the imaging means, and the power of the refracting portion and the diffracting portion is set so as to satisfy a desired condition, whereby a magnification change and focus change in the main scanning direction resulting a fluctuation in the temperature of the scanning optical apparatus may be corrected by a change in the power of the refracting portion and diffracting portion of the imaging means and a fluctuation in the wavelength of a semiconductor laser which is light source means. Thereby, even when the temperature fluctuates, it becomes possible to obtain a highly definite image.
The diffracting surface of the optical element having the refracting surface and the diffracting surface on the imaging means is usually formed into such a grating shape that as diffracted light of an order used (diffracted light used), diffracted light of the first order is of maximum intensity. Of diffracted lights diffracted by the diffracting surface at this time, unnecessary (high-order) diffracted lights of the other orders are slight in quantity relative to diffracted light of an order used to form a spot on a surfaced to be scanned. However, in a scanning optical apparatus wherein the angle of incidence onto the diffracting surface is changed by image height, the unnecessary diffracted lights are increased or decreased by the image height. Also in actual manufacture, a manufacturing error occurs to an ideal diffraction grating shape and therefore the unnecessary diffracted light may sometimes be increased.
When such increased unnecessary diffracted lights are incident on the surface to be scanned, they will become flare as stray light and will become a factor which will adversely affect the quality of image.
Further, the imaging means (scanning lens system) of the scanning optical apparatus including such a diffraction optical element is generally produced as a plastic lens and suffers from many problems technically and in cost and is therefore in a tendency to omit an anti-reflection coat provided on the refracting surface. This leads to the problem that unnecessary reflected and diffracted light created by the diffracting surface is reflected by the refracting surface of a plastic lens in which the anti-reflection coat is omitted and is incident on the surface to be scanned and becomes ghost as the stray light of the unnecessary diffracted light.
This state will hereinafter be described with reference to FIGS. 8 and 9 of the accompanying drawings.
FIG. 8 is a cross-sectional view of the essential portions of a conventional scanning optical apparatus in the main scanning direction (a main scanning cross-sectional view).
In FIG. 8, a divergent beam emitted from light source means 91 is made into a substantially parallel beam by a collimator lens 92, and this beam is limited by a stop 93 and enters a cylindrical lens 94 having predetermined refractive power only in the sub-scanning direction. In the main scanning cross section, the substantially parallel beam having entered the cylindrical lens 94 emerges therefrom in its intact state. Also, in the sub-scanning cross section, the beam converges and is imaged as a substantially linear image on the deflecting surface (reflecting surface) 95a of a light deflector 95 comprising a polygon mirror.
The beam 15 (15P, 15U, 15L) reflected and deflected by the light deflector enters imaging means (a scanning lens system) 85 comprising a refracting optical element 81 and a diffracting optical element 82. In FIG. 8, the plastic toric lens 81 and the long diffracting optical element 82 are disposed in succession from the light deflector 95 side. The long diffracting optical element 82 is made of plastic manufactured by injection molding. Both of these optical elements have different power in the main scanning direction and the sub-scanning direction, and cause the beam from the light deflector 95 to be imaged on a surface 96 to be scanned and also correct the inclination of the deflecting surface (mirror surface) of the light deflector 95. The beam having emerged from the imaging means 85 is imaged on the surface 96 to be scanned, and optically scans on the surface 96 to be scanned in the direction of arrow B (the main scanning direction) by the light deflector 95 being rotated in the direction of arrow A to thereby effect the recording of image information.
In FIG. 8, the long diffracting optical element 82 has its incidence surface 83 comprised of a refracting surface and its emergence surface 84 comprised of a diffracting surface (diffraction grating surface). Most of the beam 15 (15P, 15U, 15L) reflected and diffracted by the light deflector 95 is imaged as diffracted light used (usually diffracted light of+first order) on the surface 96 to be scanned and forms a beam spot (not shown).
However, part of the beam 15 (15P, 15U, 15L) reflected and deflected by the light deflector 95 becomes unnecessary diffracted lights of high orders. Attention is paid to reflected diffracted light of the sixth order (reflected sixth-order diffracted light) diffracted by the diffracting surface 84.
In FIG. 8, reference character 16 (16P, 16U, 16L) designates a beam (stray light of unnecessary diffracted light) of the reflected sixth-order diffracted light which is surface-reflected by the refracting surface 83 and further traveling toward the surface 96 to be scanned as diffracted light used (usually diffracted light of the+first order) by the diffracting surface 84. In FIG. 8, it will be seen that such reflected sixth-order diffracted light, although not imaged, is incident on the surface 96 to be scanned as the stray light of the unnecessary diffracted light.
How the stray light of this reflected sixth-order diffracted light scans on the surface to be scanned will now be described with reference to FIG. 9. In FIG. 9, the axis of abscissas represents the image height at which an original beam spot arrives at the surface 96 to be scanned, and the axis of ordinates represents the position in which the stray light of the then reflected sixth-order diffracted light arrives at the surface 96 to be scanned. According to this, it will be seen that as the original beam spot scans on the surface 96 to be scanned, the stray light of the reflected sixth-order diffracted light also scans on the surface 96 to be scanned correspondingly thereto, but the scanning speed falls at image heights of about ±80 mm. Thus, more stray light gathers at the image heights of about ±80 mm, and the deterioration of the quality of image becomes remarkable.
Stray light such as flare or ghost makes an image on the surface to be scanned unclear, and for example, in a laser beam printer (LBP), this leads to the problem that print becomes unclear. Further, in recent years, in order to express images having a halftone, the sensitivity of a photosensitive drum has been in a tendency toward improvement, and the deterioration of the quality of image by the stray light has become unnegligible.